[Sourceforge] There is nothing in the boost 1.71.0 directory

[tt4g]

Boost can be downloaded from Bintray and SourceForge, but There is nothing in the Boost 1.71.0 directory in SourceForge: https://sourceforge.net/projects/boost/files/boost/1.71.0/

If Bintray reaches its monthly download limit and becomes unavailable (#298, #299, #375, #383, #403), we can not download boost 1. 71.0.

We need SourceForge as an alternative download URL when Bintray is not available.